Proof readily follow from (REF ) with \(K=1\) .

It is worth noting that (REF ) is a general form of the well-known I-MMSE relationship reported in [1]}. Note that if \(\gamma \)  is considered as the square root of Signal-to-Noise Ratio (SNR), then the I-MMSE formula in [1]} will be recovered exactly. In the following
section, we show how the introduced notion of the derivative of mutual information can give more insights to the behavior of the mutual
information in a Gaussian MAC in the presence of feedback.
